Thermoresponsive Block Copolymer Core–Shell Nanoparticles with Tunable Flow Behavior in Porous Media

With the purpose of investigating new polymeric materials as potential flow modifiers for their future application in enhanced oil recovery (EOR), a series of amphiphilic poly(di(ethylene glycol) methyl ether methacrylate-co-oligo(ethylene glycol) methyl ether methacrylate) [P(DEGMA-co-OEGMA)]-based core–shell nanoparticles were prepared by aqueous reversible addition–fragmentation chain transfer-mediated polymerization-induced self-assembly. The developed nano-objects were shown to be thermoresponsive, demonstrating a reversible lower-critical solution temperature (LCST)-type phase transition with increasing solution temperature. Characterization of their thermoresponsive nature by variable-temperature UV–vis and dynamic light scattering analyses revealed that these particles reversibly aggregate when heated above their LCST and that the critical transition temperature could be accurately tuned by simply altering the molar ratio of core-forming monomers. Sandpack experiments were conducted to evaluate their pore-blocking performance at low flow rates in a porous medium heated at temperatures above their LCST. This analysis revealed that particles aggregated in the sandpack column and caused pore blockage with a significant reduction in the porous medium permeability. The developed aggregates and the increased pressure generated by the blockage were found to remain stable under the injection of brine and were observed to rapidly dissipate upon reducing the temperature below the LCST of each formulation. Further investigation by double-column sandpack analysis showed that the blockage was able to reform when re-heated and tracked the thermal front. Moreover, the rate of blockage formation was observed to be slower when the LCST of the injected particles was higher. Our investigation is expected to pave the way for the design of “smart” and versatile polymer technologies for EOR applications in future studies.

°C at a heating rate of 1 ºC min -1 (λ = 550 nm). Samples were prepared at a concentration of 5 mg mL -1 in 0.3 M NaCl(aq).

Transmission Electron Microscopy. Dry-state stained transmission electron microscopy (TEM)
imaging was performed on a JEOL JEM-1400 microscope at an acceleration voltage of 80 kV. All samples were diluted with 0.3 M NaCl (aq) to appropriate analysis concentration and then deposited onto formvar-coated copper grids. After approximately 1 min, excess sample was blotted from the grid and the grid was stained using an aqueous 1 wt% uranyl acetate (UA) solution for 1 min prior to blotting, drying and microscopic analysis. Average particle diameters (Dave) were determined by measuring at least 100 particles per sample using the ImageJ software.
pH Measurements. pH Measurements were performed using a Mettler Toledo G20 compact titrator equipped with a DGi115-SC pH-electrode. The calibration of the glass pH-electrode was performed using buffer solutions at pH = 4.01, 7.00, and 9.21, obtained from Mettler Toledo. All data were recorded using the LabX Light Titration 3.1.1.0 software provided by Mettler Toledo.
Sandpack Analysis. Sandpack testing was performed on an in-house built system ( Figure S1).
The custom-made sandpack apparatus was set up to simulate linear flow and to study polymer pore blocking experiments. A glass column of 35 cm length and 6.6 mm internal diameter was filled with sand (45 -65 μm). The column was saturated with solvent using a Strata DCP50 pump.
Pressure drop across the column was measured with a differential pressure transducer and the permeability of the sandpack was calculated using Darcy's law. The mobile phase used was 0.3 M

S6
NaCl brine adjusted to pH = 5.5 containing 0.5% ProClin™ 300 biocide. During each experiment, a flow rate of 0.1 mL min -1 was maintained and temperature was set to 90 ºC using heating tape around the column. The column was pre-heated prior to every sample injection. The eluent was monitored by UV (λ = 290 nm). All pieces of equipment were connected using 1 mm ID, 1.6 mm OD tubing. To measure the pore volume, NaI was dissolved in water to make a 0.01 mg mL -1 solution. The injection loop was filled with 2 mL NaI solution using a syringe. The injection loop was filled with 2-3 times the injection loop volume. The pump was run for enough time to ensure stabilization of the pressure and RI/UV detector. Then, the injection valve was switched on allowing for passage of the NaI solution into the column and the time was recorded from that moment by concurrent monitoring of the UV signal. The time between the injection and the signal increase multiplied by the flow rate gave the corresponding pore volume. This process was also repeated once without the column to subtract the contribution of the rest of the system. Figure S1: Schematic representation of the custom-made sandpack apparatus used in this study.

Evaluation of oligomer hydrophobicity
LogPoct Analysis. Octanol-water partition coefficients (LogP oct ) were calculated for oligomeric models (10-mers) in Materials Studio 2020, using an atom-based approach (ALogP method) for all molecular models containing C, H, and O atoms.

Surface Area Analysis.
Octanol-water partition coefficients (LogPoct) were normalized by solvent accessible surface area (SA) using Materials Studio 2020. First, oligomers were subjected to a Geometry Optimization procedure using the Forcite Molecular Dynamics (MD) module with a COMPASS II force field. The force field contains information on important parameters, like preferred bond lengths, bond angles, torsion angles, partial charges, and van der Waals radii that influence the conformation. To minimize energy and determine a preferred conformation, these simulations ran until the energy of the oligomer decreased below predetermined convergence criteria (1 × 10 -4 kcal mol -1 energy convergence, 0.005 kcal mol -1 /Å force convergence, and 5 × 10 -5 Å displacement convergence). Second, these SA values represent solvent accessible surface area created by an algorithm that rolls a ball over the surface of the oligomer. To ensure the SA values are meaningful in the context of octanol-water partition coefficients (LogPoct), the probe had a 1.4 Å radius to match the size of a water molecule.
Models. Scheme S1 depicts a representative example of P(DEGMA-co-OEGMA)-based (Px) 10mers containing x = 0, 10, 20, 30 and 40 mol% of OEGMA units. To simplify calculations, blocky oligomers with a consistent cis conformation were selected for analysis in all cases (an all-trans conformation did not affect calculated LogPoct/SA values).   histogram of their corresponding size distribution along with calculated average diameter, measured from particle analysis based on acquired dry-state TEM images.